Global change and contaminants in the Arctic

May 7, 2012, 1:28 pm
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This is Section 8.7 of the Arctic Climate Impact Assessment Lead Authors: Frederick J.Wrona,Terry D. Prowse, James D. Reist; Contributing Authors: Richard Beamish, John J. Gibson, John Hobbie, Erik Jeppesen, Jackie King, Guenter Koeck, Atte Korhola, Lucie Lévesque, Robie Macdonald, Michael Power,Vladimir Skvortsov,Warwick Vincent; Consulting Authors: Robert Clark, Brian Dempson, David Lean, Hannu Lehtonen, Sofia Perin, Richard Pienitz, Milla Rautio, John Smol, Ross Tallman, Alexander Zhulidov

 

During the past 50 years, persistent organic pollutants (POPs), metals, and radionuclides have been widely distributed into northern freshwater ecosystems by long-range atmospheric transport[3]. Within some catchments, deposition from the atmosphere may be augmented locally by industry or agriculture[4] or biotransport[5]. Figure 8.21 illustrates contaminant pathways, transfers, and exchanges in freshwater systems. This section briefly discusses how projected global change might alter these pathways, focusing especially on POPs and Hg because they have the greatest potential for change in risks to freshwater ecosystems as a result of climate change[6].

 

caption Fig. 8.21. Contaminant pathways, transfers, and exchanges in freshwater systems: land–atmosphere–water interactions[1].

 

Contaminant pathways and arctic freshwater ecosystems (8.7.1)

There are two components of long-range transport pathways: transport to arctic freshwater catchments, and processes within the catchments (Fig. 8.22). Transports and transfers within each of these components are altered by climate change manifested in such things as wind fields, precipitation (amount, timing, form), snow cover, permafrost, extreme events, UV radiation exposure, the hydrological cycle, ice cover, the organic carbon cycle, and food webs, and are very likely to result in enhanced bioaccumulation of contaminants (Box 8.12).

 

caption Fig. 8.22. Processes involved in transporting POPs to the Arctic and depositing them into terrestrial ecosystems. Transport, deposition, and exchange can occur anywhere along the transport pathway. Contaminants can also be transported within aquatic and terrestrial food chains. Climate change can alter the physical couplings between the systems (e.g., by changing rain or snowfall patterns), or alter the biological couplings by changing trophic structure or migratory pathways.

 

Before describing specifically how global change may alter contaminant pathways, it is important to understand how contaminants become concentrated in the environment. Macdonald R. et al.[7] suggest that there are two distinct concentrating processes, which they term solvent switching and solvent depletion. Solvent switching can, for example, lead spontaneously to concentration amplification of hexachlorocyclohexanes (HCHs) in water because HCH partitions strongly out of air[8], or high concentrations of PCBs in phytoplankton due to strong partitioning out of water and into lipids[9]. Solvent depletion involves a reduction in the mass of solvent in which the contaminant is held, a process that can lead to fugacity amplification (i.e., POP concentrations exceeding thermodynamic equilibrium with the surrounding media). Examples include inefficient fat transfers in aquatic food webs (i.e., biomagnification)[10], the loss of organic carbon in settling particles or during sediment diagenesis[11], the decrease of snow surface area as crystals become more compact during aging or the entire loss of snow surface during melting[12], or cryogenic concentration during the formation of ice[13] (Fig. 8.23). Many of the consequences of climate change for the solvent-switching processes are relatively easy to project and model because the effect of temperature on partition coefficients is known. For example, for contaminants that presently are saturated in arctic surface waters, increased temperatures will generally lead to net gas evasion[14] and the ocean is very likely to become a net source of those contaminants to the atmosphere. McKone et al.[15] concluded that, with temperature increases, the risk hexachlorobenzene (HCB) presents to aquatic biota is likely to decrease slightly because HCB will partition less into water[16]. The solvent-depleting processes, however, provide a much greater challenge to projection and have not yet been incorporated realistically into models.

 

caption Fig. 8.23. In the illustrated solvent-depleting processes, POPs are concentrated beyond thermodynamic equilibrium through the removal of solvent by organic carbon metabolism in (a) aquatic and (b) terrestrial carbon cycles, by (c) inefficient lipid transfer in aquatic food webs, by (d) exclusion into a dwindling layer of water during the growth of ice, by (e) the loss of snow surfaces during aging or melting, by (f) the loss of surface area through condensation of fog into water droplets or onto surfaces, and (g) through loss of lipid pools during periods of starvation.

 

Persistent organic pollutants in arctic catchments (8.7.2)

The freeze, melt, and hydrological cycles and the organic carbon cycles of arctic lakes are likely to provide sensitive sentinels of change. As discussed in more detail in the previous sections and Chapter 6, probable changes as a result of projected temperature increases include reduced thermal contrast between winter and spring; reduced duration of snowmelt (later freeze-up, earlier melting); reduced ice formation; increased annual precipitation; thawing of permafrost producing a deeper active layer, enhanced soil erosion, mobilization of organic carbon, and reduced pond areas owing to drainage; more frequent extreme weather events; changes in catchment vegetation (i.e., more leaf-bearing plants); changes in nutrient availability; warming of lakes; and an increase in the frequency of wildfires[17]. For some lakes, permafrost degradation together with reduced ice cover is very likely to result in enhanced nutrient and organic carbon loadings and higher productivity. Conversely, if dry summer conditions produce extensive fires, affected lakes are very likely to receive reduced spring melt, fewer nutrients from the catchment, reductions in productivity[18], and higher burdens of combustion PAHs.

Most arctic lakes receive their contaminant burdens from the atmosphere, with the catchment acting as a receptor through snow, rain, and dry deposition especially during winter, and a conveyor through snow and ice melt and runoff in spring[19]. This section describes the stages from contaminant release to its final emergence in top freshwater predators (Fig. 8.22), noting in particular those components of the pathway likely to be altered as a result of climate change.

Upon release, contaminants are transported through the atmosphere either as gases or adsorbed onto particles. During atmospheric transport, washout and air–surface exchange remove some of the contaminant to the surface where it may become permanently sequestered or re-volatilized as a result of, for example, seasonal heating cycles, eventually arriving in the Arctic via a number of "hops"[20]. Accordingly, POPs undergo hemispheric-scale chromatography, with surfaces (soil, water, vegetation) providing the stationary phase and the atmosphere providing the moving phase. Global temperature increases will generally accelerate this cycling. Processes that are effective at capturing contaminants in arctic drainage basins (e.g., strong partitioning onto particles, into precipitation, into vegetation) are also effective at removing contaminants during transport. For example, Li et al.[21] suggested that air–water partitioning alone restricts the entry of betahexachlorocyclohexane (?-HCH) into the Arctic by removing it to surfaces by rain and air–sea exchange. In the case of ?-HCH, and in the case of contaminants that partition strongly onto particles (e.g., many PAHs, dichloro-diphenyltrichloroethane (DDT), and highly chlorinated PCBs), changes in rainfall patterns (amount and location) are very likely to alter the efficiency of transport to arctic locations and capture within the Arctic. Heavy metals provide an instructive example that will likewise apply to many of the POPs. Presently, less than 20% of particulate metal entering the Arctic is captured there[22]. Since the five ACIA-designated models project that the Arctic will become a "wetter" place[23] (section 4.4.3), the capture of particulates and contaminants that partition strongly into water is likely to significantly increase by a factor that could more than offset efforts to reduce global emissions.

Box 8.12.Temperature-induced metal accumulation and stress responses in fish from Canadian Arctic lakes

High-altitude and high-latitude lakes are very sensitive ecosystems, where even slight environmental changes will possibly substantially affect ecosystem function[24]. Environmental changes can alter fish habitat and toxicant accumulation from water and diet. Long-range transport of pollutants also tends to endanger fish populations by leading to highly elevated metal accumulation[25]. Water temperature has been shown to be the driving force of excessive metal accumulation in these fish. A multi-year project, centered around small sensitive lake ecosystems in the Canadian Arctic Archipelago (Cornwallis Island, Somerset Island, and Devon Island), was designed to explain the interactions between short- or longer- term climatic variation, the bioaccumulation of metals, and various biochemical stress indicators in land-locked populations of Arctic char.

 

caption Interannual variation of the GSSG/GSH ratio in the liver of Arctic char (n=20 individuals per group) from Resolute Lake, showing means ± standard deviation[2].

 

Arctic char were collected at monthly intervals from Resolute Lake (Cornwallis Island, Nunavut, 74º 41' N, 94º 57' W) during summers from 1997 to 2001. Fish were dissected and liver tissue subsampled for analysis of metal content (cadmium – Cd and zinc – Zn) and
biochemical stress indicators (glutathione – GSH, glutathione disulfide – GSSG, glycogen, and Vitamin C). Glutathione is an antioxidant, which is reduced to GSSG in the presence of reactive oxygen species. The GSSG/GSH ratio is a sensitive indicator of oxidative stress in cells[26]. Stress response was indicated by a decrease in GSH accompanied by an increase in GSSG.

Similar to Arctic char from Austrian high-mountain lakes, concentrations of Cd and Zn in the liver of high-latitude char exhibited a marked seasonal change during summer, and were significantly higher at the end of the icefree period.A similar pattern was found for concentrations of metallothione (an inducible metal-binding protein) in the liver. Concentrations of Cd and Zn in the liver of char collected in August 1998 were significantly higher than those in fish collected during the same period in 1997, 1999, 2000, and 2001, which coincides with much higher lake water temperatures in the El Niño year of 1998[27]. Cadmium concentrations in the liver were positively correlated with frequencies of high temperatures (4–12 ºC) and negatively with those of lower temperatures (<0–4 ºC). Interannual variation of the GSSG/GSH ratio, glycogen, and Vitamin C levels in the char studied indicate a higher level of stress in 1998 than in 1997 and 1999: the GSSG/GSH ratios in the livers of fish collected in 1998 were significantly higher than in 1997 and 1999 (see figure). Furthermore, concentrations of glycogen and Vitamin C were significantly lower in 1998.The severe depletion of glycogen energy reserves indicates that atypically high lake temperatures enhanced metal bioaccumulation and detoxification responses, diverting energy resources from other important physiological functions.

These results illustrate that Arctic char are extremely susceptible to even slight changes in lake water temperatures. Rising water temperatures lead to increased metabolic rates and thus pumping of higher volumes of water across the gills, which in turn results in increased uptake of dissolved metals from the water.The rapid increase in temperatures projected by various GCMs will possibly be a serious threat to the stability of Arctic char populations in high-latitude lakes.

 

Because much of the contaminant delivery to the Arctic occurs during late winter as "arctic haze" or as "brown snow" events[28], it is clear that sequestering by snow is an important process. Hence, careful consideration must be given to any changes in arctic snow conditions. Newly formed snow has a large surface area (as much as 0.4 m2/g[29]) that scavenges both particulate and gaseous POPs, eventually sequestering them into the snowpack[30]. Precipitation form (snow, rain, fog) is therefore important and, considering the seasonal modulation in atmospheric concentrations of contaminants[31], so is timing. For example, snowfall during a period of arctic haze would be much more important for transferring contaminants to the ground than at other times of the year.

As snow ages or melts, its surface area to volume ratio decreases, resulting in the removal of the solvent that captured the POPs[32]. Macdonald R. et al.[33] estimated that this process could lead to fugacity amplification of ~2,000 times that of the air –clearly an enormous thermodynamic forcing. Depending on the exact circumstances under which snow loses its surface area, the POPs will be vaporized back to the air or partitioned into particles, soil, vegetation, or meltwater. Changes in the frequency and timing of snowfall, unusual events like freezing rain, or the rate and timing of snowmelt are likely to effect large changes in the proportion of POPs that enter the arctic hydrological cycle.

Terrestrial organic carbon in soils and vegetation has a large capacity to store many POPs[34], with PCBs, DDT, HCH, and chlorobenzenes figuring prominently[35]. Wania and McLachlan[36] have shown that forests "pump" organochlorines from the atmosphere into foliage and subsequently to long-term soil reservoirs. Accordingly, increased proportions of leaf-bearing plants in arctic catchments will enhance this "pump". Increased metabolism of soil organic carbon owing to temperature increases, changes in soil moisture, or reduced snow cover will force POPs associated with soil organic carbon to redistribute, probably into groundwater or meltwater.

Climate variation results in the storage of contaminants in perennial snow and ice or in soils, vegetation, and delta/estuarine sediments during periods of cold climate (years to decades). These stored contaminants may subsequently be released during a period of warming and, although this process may not be sustainable, it is likely to produce episodes of high contaminant loadings into water[37]. During permafrost degradation, a shift toward dendritic drainage patterns[38] allows a more complete transport of contaminants into ponds and lakes and possibly re-mobilizes contaminants stored in soils. Simultaneously, the reduction of pond areas owing to drainage channels in permafrost[39] is likely to enhance contaminant transport into the remaining surface water.

After POPs enter the hydrological cycle through the mechanisms discussed, a proportion of them will be stored in lakes and lake sediments. Evidence from a limited number of studies[40] suggests that meltwater currently enters arctic lakes when they are thermally stratified beneath an ice cover. Therefore, much of the annual snowmelt traverses under the ice to exit at the outflow carrying its contaminant burden; that is, arctic lakes are not efficient at capturing POPs entering via streamflow. It is probable that many of the previously described alterations in freshwater systems induced by climate change (such as reduced ice cover, increased mixing and primary production, and increased loading of organic carbon and sediment from the contributing catchments) will also enhance contaminant capture in lakes[41].

Because most arctic lakes tend to be oligotrophic, only a small proportion of POPs is transported by vertical flux of organic particles and buried in sediments[42]. A second solvent-depletion process occurs due to organic carbon metabolism during particle settling and within bottom sediments (Fig. 8.23a). The loss of organic carbon can provide exceptionally strong thermodynamic forcing to drive the POPs off solid phases and into sediment pore water, where they may diffuse into bottom waters or partition into benthos[43]. An increase in the vigor of the organic carbon cycle (e.g., increased primary production, organic carbon loadings, and microbial activity) will enhance this thermodynamic pump. Cryogenic concentration (Fig. 8.23d) is likely to work together with organic carbon metabolism in sediments during winter to produce exceptionally high concentrations of POPs in bottom water. Although relatively poorly studied, contaminants are believed to be excluded from ice as it forms. For shallow water that freezes nearly to the bottom, dissolved contaminants are likely to be forced into a very small volume of remaining water and the resultant high contaminant concentrations will promote partitioning into remaining organic material including sediment surfaces, benthos, plankton, and larger animals. As noted previously, such under-ice zones are often a critical winter refuge for biota[44]. It is likely that a general reduction in the depth of ice formed during warmer winters will reduce cryogenic concentration. However, cryogenic concentration interacts with water levels, which are likely to decrease during permafrost degradation[45].

The transfer of lipid-soluble POPs upward in aquatic food webs is one of the most important routes of exposure to apex feeders, including humans. In this solvent-depleting process, much of the lipid is metabolized at each trophic level while the organochlorines are retained. This results in higher trophic levels exhibiting organic carbon biomagnification factors of 3 to 100 in their lipids, and a net bioaccumulation of 107 to 109 times higher than in the water[46]. Changes in aquatic trophic structure either through alteration of the number of food web steps or the size distribution of predatory fish will likewise change contaminant burdens. With climate change, wide-ranging shifts in zoogeographic distributions have the potential to affect every step in freshwater food chains[47].

There are several other ways that global change can alter contaminant pathways in arctic aquatic ecosystems. As noted in section 8.5.4, recent evidence suggests that salmon migrations undergo large, climate-related variation[48] and that Pacific salmon may respond to change by invading arctic rivers[49]. Given that these salmon biomagnify and bioaccumulate contaminants in the Pacific Ocean, they are an important means of contaminant transport into particular arctic catchments. In specific lakes, fish may supply more POPs than atmospheric deposition[50]. Similarly, bird migrations that change in location and intensity have the potential to concentrate, transport, and deposit contaminants in particular catchments[51]. For example, detailed studies of Lake Ellasjoen, Norway, found that seabirds can serve as important biological pathways carrying contaminants (in this case POPs) from marine to freshwater environments[52]. Climate change or human intervention is also very likely to lead to the introduction of exotic species to the Arctic. Although probably not a risk to arctic lakes, the invasion of the Great Lakes by the zebra mussel (Dreissena polymorpha) provides an instructive example of just how disruptive an exotic species can be to organic carbon and POP cycles[53].

Changes within arctic catchments that cause apex feeders (e.g., humans, bears, mink, birds) to switch their diet from aquatic to terrestrial food sources or vice versa have a large potential to alter contaminant exposure. Whereas arctic aquatic food webs exhibit endemic contamination from biomagnifying chemicals, arctic land-based food webs are among the cleanest in the world[54]. Dietary changes are forced by fluctuations in the populations of prey species or by changes in access to the species due to early ice melt or permafrost degradation[55].

As conditions more suitable for domestic crops develop, agriculture or silviculture within arctic drainage basins and associated chemical use is likely to expand. Demographic shifts and population increases in northern regions could possibly lead to increased local release of contaminants. South of the Arctic, global temperature increases and alteration of hydrological cycles will probably result in insect and other pest outbreaks (e.g.,West Nile virus or malaria), provoking the re-introduction of banned pesticides[56]. Finally, contaminants in dumps or sumps presently contained by permafrost are very likely to be released by permafrost degradation[57].

Increased fluxes of PAHs are likely to result from the erosion of peat-rich soils[58] or drying trends leading to an increase in wildfires, and are likely to have a greater impact on small rivers that presently receive most of their PAHs from combustion sources[59].

Mercury in arctic catchments (8.7.3)

Mercury exhibits a natural global cycle that has been enhanced by human activities such as coal burning, soft and ferrous metal smelting, cement production, and municipal waste with the consequence that two to three times as much Hg is now cycling through the atmosphere and surface waters than was before the rise of industry[60]. Pacyna and Pacyna[61] estimated that worldwide anthropogenic Hg emissions totaled 2,235 x 103 kg in 1995, with fossil fuel consumption contributing over half of that. This value may be compared to the 2,500 x 103 kg/yr estimate of natural emissions[62]. The largest emitter of Hg from fossil fuel consumption is China (495 x 103 kg in 1995), which is directly upwind from the Bering Sea, Alaska, and the western Arctic. In comparison, Russia released about 54 x 103 kg in 1995.

 

caption Fig. 8.24. Production of particulate and reactive gaseous mercury over the ocean after polar sunrise (right side) and the advection of reactive and bioavailable forms of mercury into catchments where it is deposited. After deposition, the mercury enters lakes through meltwater and is then subject to reduction and methylation (meth.) processes. Methyl mercury (CH3Hg(II)) is the most toxic form.

 

Many of the concentrating processes discussed for POPs (Fig. 8.23) apply equally to Hg. However, atmospheric Hg depletion events (MDEs) after polar sunrise provide a unique, climate-sensitive pathway to Hg deposition in arctic catchments[63] (Fig. 8.24). The process requires snow surfaces, solar radiation, and the presence of sea salts (bromides and chlorides). Although MDEs are initiated over the ocean, and especially over the marginal seas where halides are more available from frost flowers (crystals of ice that form directly from the vapor phase, often associated with new sea ice, which can become very salty by channeling brine upward from the ice) or first-year ice[64], atmospheric advection can subsequently deposit reactive Hg in arctic catchments[65]. As with POPs, Hg can be transferred and concentrated during snow aging and melting, such that a large pulse of Hg is released to terrestrial and freshwater environments during spring melt[66]. Scott K.[67] showed that Hg deposited through the MDE mechanism is in a form that can readily be taken up by biota. Once Hg enters the hydrological cycle, it can be concentrated and transferred through the carbon cycle and food webs, both of which are vulnerable to change. In addition, the efficiency of arctic lakes in capturing Hg is very likely to be altered by changes in the timing of freshet, ice melt, and productivity[68].

Once Hg has been deposited into arctic catchments, a number of processes may lead to elevated concentrations in old, predatory fish (Table 8.5). The coupling between Hg deposition on surfaces and its entry into lakes is likely to be enhanced by projected changes in the hydrological and organic carbon cycles[69]. Apex feeders are most vulnerable to any change in the Hg cycle considering that biomagnification factors are 250 to 3,000[70]. Because MeHg presents a far greater health hazard than inorganic or elemental Hg, methylation is a crucial process upon which climate change operates. Wetlands and wetland sediments are net producers of MeHg[71], and Hg observed in fish from small lakes appears to correlate with the amount of watershed occupied by wetlands[72]. Flooding of terrestrial landscapes has the well-known consequence of releasing Hg from submerged soils[73]. Therefore, alteration of wetland distribution or area in the Arctic resulting from thawing permafrost (section 8.4.4.4) is very likely to release Hg, which will be more serious if arctic soils contain an inventory of contaminant Hg accumulated during the past century or two.

Table 8.5. Environmental factors affecting mercury concentration in aquatic top predators.

Projected change

Effect on mercury concentration in predatory fish

Reference

Flooding of soil

Increase

[74]

Increased primary production

Reduce

[75]

Increased number of trophic levels

Increase

[76]

Shift toward larger fish

Increase

[77]

Reduced lake size

Increase

[78]

Increased anadromous fish migration

Increase

[79]

 

 Chapter 8: Freshwater Ecosystems and Fisheries
8.1. Introduction
8.2. Freshwater ecosystems in the Arctic
8.3. Historical changes in freshwater ecosystems
8.4. Climate change effects
        8.4.1. Broad-scale effects on freshwater systems
        8.4.2. Effects on hydro-ecology of contributing basins
        8.4.3. Effects on general hydro-ecology
        8.4.4. Changes in aquatic biota and ecosystem structure and function
8.5. Climate change effects on arctic fish, fisheries, and aquatic wildlife
        8.5.1. Information required to project responses of arctic fish
        8.5.2. Approaches to projecting climate change effects on arctic fish populations
        8.5.3. Climate change effects on arctic freshwater fish populations
        8.5.4. Effects of climate change on arctic anadromous fish
        8.5.5. Impacts on arctic freshwater and anadromous fisheries
        8.5.6. Impacts on aquatic birds and mammals
8.6. Ultraviolet radiation effects on freshwater ecosystems
8.7. Global change and contaminants
8.8. Key findings, science gaps, and recommendations

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Citation

Committee, I. (2012). Global change and contaminants in the Arctic. Retrieved from http://www.eoearth.org/view/article/153019

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